refactor(physics): Phase 2 — code-review polish on BuildingPhysics commit

Five reviewer-flagged items addressed:

- Fix #1: GameWindow building-loop now reuses TerrainSurface.ComputeOutdoorCellId
  instead of re-deriving the row-major cell-index formula. DRY win; no risk
  of the two formulas drifting.
- Fix #2: BuildingPhysics.ExactMatch decoder now references
  DatReaderWriter.Enums.PortalFlags.ExactMatch instead of magic 0x0001.
- Fix #3: ExactMatch XML doc clarified as "reserved per retail's
  CBldPortal::exact_match; not currently consumed by CheckBuildingTransit".
- Fix #4: CheckBuildingTransit docstring now explicitly documents the
  retail divergence — retail's sphere_intersects_cell (radius-aware) vs.
  our PointInsideCellBsp (radius-less). The sphereRadius parameter is
  reserved for the future sphere_intersects_cell port. Practical effect
  noted: entry fires ~sphereRadius (~0.48m) deeper than retail.
- Fix #5: Test method `SphereInsideBuildingPortalDestination_AddsInteriorCell`
  renamed to `BuildingPortalWithUnloadedCellBSP_NoCandidateAdded` — the
  test asserts Empty(candidates), not that the cell is added. Comment
  updated.

Spec: docs/superpowers/specs/2026-05-19-indoor-portal-cell-tracking-design.md
Plan: docs/superpowers/plans/2026-05-19-indoor-portal-cell-tracking.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/BuildingPhysics.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Numerics;
+using DatReaderWriter.Enums;
 
 namespace AcDream.Core.Physics;
 
@@ -36,6 +37,16 @@ public readonly struct BldPortalInfo
     public ushort OtherPortalId { get; }
     public ushort Flags         { get; }
 
-    /// <summary>Bit 0 of Flags (<c>PortalFlags.ExactMatch = 0x0001</c>).</summary>
-    public bool ExactMatch => (Flags & 0x0001) != 0;
+    /// <summary>
+    /// Bit 0 of <see cref="Flags"/> (<c>DatReaderWriter.Enums.PortalFlags.ExactMatch</c>).
+    ///
+    /// <para>
+    /// Reserved per retail's <c>CBldPortal::exact_match</c>. NOT currently
+    /// consumed by <see cref="CellTransit.CheckBuildingTransit"/> — every
+    /// portal overlap is treated as a valid entry trigger. If a future
+    /// regression surfaces (e.g., a building entered by overlapping a
+    /// non-exact-match portal), wire this into the entry test.
+    /// </para>
+    /// </summary>
+    public bool ExactMatch => (Flags & (ushort)PortalFlags.ExactMatch) != 0;
 }